The present invention relates to the superconducting electronics field. More specifically, the present invention relates to a superconducting driver circuit of a superconducting flux quantum circuit which uses a flux quantum permitting high-speed signal processing as the carrier of a signal and is used for a measuring circuit for high-speed signal observation, an analog-to-digital signal converter circuit for high-speed analog signal processing, or a high-speed digital data processing circuit.
Two types of prior art driver circuits which are used for the output part of a superconducting flux quantum circuit and have a signal voltage amplifying function are considered and are used in a superconducting circuit. The prior art and embodiments will be described below with reference to the drawings. Only when discriminating between the parts indicated by the reference numerals, numerical subscripts will be identified as needed.
FIG. 2 is a diagram showing a constructional example of a squid type superconducting driver circuit with a control line which has been used. Superconducting flux quantum interference devices each having a closed loop by superconducting junctions 1 and an inductor 7, that is, SQUIDs 6 are connected in series and have a control line 21. A signal line inputting a signal 22 from a superconducting flux quantum circuit is connected to the control line 21 of the SQUIDs connected in series. The numeral 3 denotes a bias current source; the numeral 5 denotes an output line; and the numeral 8 denotes a ground.
There are two control line wiring methods. In one of the methods, one superconducting line is wired to SQUIDs in series as the control line of all the SQUIDs. In the other method, one signal line is branched to wire control lines to SQUIDs in parallel. When connecting the control lines in parallel, a signal is inputted to the SQUIDs at the same time, thereby enhancing the frequency characteristic.
The flux quantum signal 22 from the superconducting flux quantum circuit is inputted as a current signal to the control line 21. The output voltages of the SQUIDs 6 are changed by the current signal. A change in output voltage per SQUID is a small value of about 0.1 mV. To increase a change in voltage, ten or more squids are connected in series (“Josephson Output Interfaces for RSFQ Circuits” O. A. Mukhanov et al., IEEE Transactions on Applied Superconductivity, vol. 7, p. 2826, 1997).
FIG. 3 is a diagram showing another constructional example of a superconducting driver circuit using superconducting junction lines which has been used. One end of a superconducting junction line 1001 and one end of a superconducting junction line 1002 connecting superconducting junctions 1 in series are connected in parallel via resistances 31 and an inductor 9. An AC power source 32 supplies an AC voltage to the midpoint of the inductor 9. An output signal 5 is fetched from the midpoint of the inductor 9. The other end of the superconducting junction line 1001 and the other end of the superconducting junction line 1002 are grounded. The first stage of the superconducting junction line 1001 is connected to an input line to input a flux quantum signal 22. In the superconducting driver circuit, a superconducting junction exhibiting the hysteresis characteristic in the current-voltage characteristic is used.
The signal current pulse 22 from a superconducting flux quantum circuit is injected into a superconducting junction 11 in the first stage of the superconducting junction line 1001. When an electric current of the signal current pulse 22 and a bias current of the AC power source 32 exceed a critical current, the superconducting junction 11 is switched from the superconducting state to the voltage state. The resistance value of the superconducting junction in the voltage state is relatively high. The bias current selectively flows to the superconducting junction line 1002. The current values of the superconducting junctions 1 constructing the superconducting junction line 1002 exceed the critical current. The superconducting junctions 1 constructing the superconducting junction line 1002 switched together from the superconducting state to the voltage state. The resistance of the superconducting junction line 1001 including one superconducting junction 11 in the voltage state is lower than that of the superconducting junction line 1002 in which the superconducting junctions 1 switched together to the voltage state. This time, the bias current exclusively flows to the superconducting junction line 1001. The remaining superconducting junctions of the superconducting junction line 1001 all switched from the superconducting state to the voltage state (“Applications of Synchronized Switching in Series-Parallel-Connected Josephson Junctions” H. Suzuki et al., IEEE Transactions on Electron Devices, vol. 37, p. 2399, 1990).
The superconducting junctions exhibiting the hysteresis characteristic are used in the superconducting driver circuit shown in FIG. 3. Unless an applied current is lower than a predetermined value, the superconducting junctions which once switched to the voltage state are not returned to the zero-voltage state. When the superconducting junction exhibiting the hysteresis characteristic switched, a voltage value is at millivolt level. It is possible to obtain an output voltage sufficiently higher than 0.1 mV to 0.5 mV as signal voltages of a flux quantum.